as_index() to coerce a matrix/data frame of index values into an index
object.
Other index methods:
[.piar_index(),
aggregate.piar_index,
as.ts.piar_index(),
chain(),
contrib(),
head.piar_index(),
is.na.piar_index(),
levels.piar_index(),
mean.piar_index,
merge.piar_index(),
split.piar_index(),
stack.piar_index(),
time.piar_index(),
window.piar_index()